Subject: A Microscopic Understanding of Exchange Anisotropy in Fe/MnF_2 Bilayers

Refreshments served in Room 216 Physics after colloquium

A microscopic understanding of the Ferromagnetic/Antiferromagnetic (F/AF) direct exchange coupling or exchange biasing has been elusive for the over 50 years since its discovery. In part, the almost exclusive use of hysteresis loops to study the phenomenon has limited our understanding. We developed a new experimental technique to study the exchange coupling between a ferromagnet and antiferromagnet [Appl. Phys. Lett. 69,3932-3931 (1996)]. This new technique enjoyed considerable success in explaining many general exchange bias features using Co/CoO as a model system [J. Appl. Phys. _87_, 6418-20 (2000), J. Appl. Phys. _89_, 7543-5, (2001), and Phys. Rev. _B 65_(RC) 180406-10(2002)]. After the Co/CoO work we used variations of the technique to study the angular dependence of the interfacial energy in Fe/MnF_2 bilayers. We were able to explain the observations using a microscopic model [Phys. Rev. _B 65_(RC), 100402, (2002), Phys. Rev. _B 68_, 054430 (2003)]. The microscopic model includes terms for the interfacial exchange coupling, uncompensated spin density in the AF, the AF spin-canting energy, and domain walls in the AF. Application of the model to the Fe/MnF_2 bilayer experimental data allows one to separately determine the fraction of uncompensated interfacial spins in the AF layer and the interfacial exchange coupling energy for the first time. An understanding of the spatial distribution of the microscopic energies allows for a simplification of the energy in which the physics is transparent.This work supported by the University of Minnesota MRSEC